Evaluation of Existing Semi-Empirical Models of Scour at a Submarine System

Assessment of scour below pipelines and gravity based structures in a submarine system is revisited with evaluation of data from two inspections and results for semi-empirical simplified models. Usually semi-empirical models are based on small scale experimental data that normally consider uniform and noncohesive sediments and have been questioned as to their practical applications. Recent local metocean data is used to revisit predictions of onset of scour and scour depth for the system, which are compared to survey statistics.

The Influence of Cohesion on Sediment Behaviour

This paper describes the results of a laboratory study financed by the Science and Engineering Research Council (SERC), UK. The work was carried out at the University of Newcastle upon Tyne in collaboration with the Water Research Centre's (WRc) River Basin Management Project during the period 1987-90. The present study has covered hydraulics, deposition, erosion and sediment transport, all with deposited bed. Noncohesive sands and sewer sediment analogues (with cohesive additives to sand) have been used and throughout the study comparisons between cohesive and noncohesive sediments were made. The noncohesive sediment studies suggested that the initiation of erosion and transport rates criteria in channels of circular cross-section using bed shear stress are comparable to those of wide channels. The sewer sediment analogues corresponding to type A needed a maximum mean shear stress of around 6-7 N/m2 whereas a weaker sediment (type C) needed only around 2.5 N/m2. The chosen cohesive analogues behaved as noncohesive sediments once they started moving, perhaps a phenomenon close to reality.